Devein the prawns, then remove the shells and heads and reserve. Place the shells and heads into a pan with the boiling water and bring to the boil, then reduce the heat and simmer for 5-10 minutes. Strain the liquid through a sieve and reserve the liquor. Discard the heads and shells.
In a separate pan, heat the oil over a low heat and add the garlic, cooking gently for 3-4 minutes. Remove the garlic from the oil, turn up the heat and add the prawns. Cook for 1-2 minutes, or until the prawns are beginning to turn pink, then stir in the chopped tomatoes and add the tomato purée.
Dissolve the stock cube in the prawn stock and pour into the pan with the prawns. Cook for 3-4 minutes, then stir in the lemon juice, parsley and season with salt and freshly ground black pepper. Stir in the cooked linguine.
Serve the linguine with a drizzle of olive oil.